Distributed generation is growing at a fast pace as society increases its use of renewable resources such as solar energy. The majority of solar power systems in use at this time make use of multiple polycrystalline silicon photovoltaic panels connected in series which are then connected to a string inverter that pumps the power back into a utility grid. The photovoltaic panels are usually placed in series to provide a high DC voltage to the string inverters.
With the proliferation of distributed generation came some problems with stability of the utility grid. As a result, most electric utilities world-wide are now seeking increased functionality in grid-tied inverters to help the inverters stabilize the grid. One such function, for example, is the introduction of VAR compensation as a function of load. To comply with new utility requirements, inverters must be able to source or sink AC current that is out-of-phase with the grid voltage.
There are several problems with string inverters. Firstly, the high DC input voltage presents safety issues as well as fire hazards, and also requires an expensive balance of system. Secondly, the use of series-connected photovoltaic panels causes all panel power to go to zero when even a single panel is shaded. This second issue leads to reduced energy harvest in most practical systems.
The sited issues of the string inverters have been addressed in recent years through the use of micro-inverters. Micro-inverters convert energy from a single photovoltaic panel into AC power that can be sourced into the utility grid. The micro-inverter is mounted under or adjacent to the photovoltaic panel. The use of a single inverter per panel reduces the DC input voltage to that of a single panel—a voltage which is typically in the range of 20V-50V for a polycrystalline silicon panel. The use of a single inverter per panel also solves the issue of power reduction when a single panel is shaded because all non-shaded panels continue to produce usable power.
Various micro-inverter topologies have been developed and brought to market. Examples can be found in WO2007/80429A2, WO2006/48688A1, and U.S. Pat. No. 7,796,412B2. Each of these topologies has shortcomings. For example, most micro-inverters use electrolytic capacitors in their design. Electrolytic capacitors have a very limited operating lifetime, particularly when operating at the high temperatures seen on rooftops (the standard place to mount a photovoltaic panel). Furthermore, most micro-inverter topologies are designed to only source current into the grid and therefore cannot accommodate new utility requirements to produce reactive power.
Micro-inverters that do not use electrolytic capacitors typically have a very high-voltage, highly variable DC bus. This high-voltage, highly variable DC bus can cause large switching losses on the power semiconductors that are connected to the bus, high voltage stress on the inverter components, and significant production of EMI.
What is needed is a micro-inverter topology that utilizes non-electrolytic capacitors for energy storage elements, that operates with lower internal voltages, and that is capable of sinking and sourcing AC current that is out-of-phase with the utility grid voltage.